Specially-formulated coatings are very useful for protecting metal parts which are exposed to high temperatures. Aircraft engines are made from such parts. The combustion gas temperatures present in the turbine engine of an aircraft are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000 C.-1150 C. Operation above these temperatures may cause the various turbine elements to fail and damage the engine.
The protective coatings, often referred to as thermal barrier coatings or "TBC"s, effectively increase the operating temperature of the turbine engine by maintaining or reducing the surface temperature of the alloys used to form the various engine components. Most TBC's are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually stabilized with another material such as yttria. For a jet engine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles.
Usually, the TBC ceramics are applied to an intervening bond layer which has been applied directly to the surface of the metal part. The bond layer is often critical for improving the adhesion between the metal substrate and the TBC. Bond layers are usually formed from a material like "MCrAlY", where "M" represents a metal like iron, nickel, or cobalt. Very often, the bond layer is applied by a plasma spray technique. There, an electric arc is typically used to heat various gasses, such as air, oxygen, nitrogen, argon, helium, or hydrogen, to temperatures of about 8000 C. or greater. (When the process is carried out in an air environment, it is often referred to as air plasma or "AP".) The gasses are expelled from an annulus at high velocity, creating a characteristic thermal plume. Powder material is fed into the plume, and the melted particles are accelerated toward the substrate being coated. Plasma-formed layers usually have a very rough surface, which enhances their adhesion to a subsequently-applied TBC. As used herein, the term "TBC system" refers to the bond coat and any other intermediate coating in combination with the TBC.
The TBC itself can be applied by a variety of techniques. One popular method is referred to as electron beam physical vapor deposition (EB-PVD). In one version of this technique, an ingot of the material being deposited is placed in a chamber which is then evacuated. The top end of the ingot is then heated by an intense heat source (i.e., the electron beam), so that it melts and forms a molten pool. A portion of the very hot, molten ceramic evaporates and deposits (condenses) on a substrate positioned above the pool. In this manner, a coating is gradually built up on the substrate, as the ingot is moved upwardly to be melted and replenish the pool.
The use of EB-PVD has many advantages. The technique is especially suited for providing high-integrity coatings for parts which are difficult to coat by other methods, e.g., turbine blades having a multitude of fine cooling holes extending into the blade structure. The consistent, columnar grain structure in a coating deposited by EB-PVD results in a good thermal match between the TBC and a substrate, e.g., a superalloy component.
However, EB-PVD is not the coating technique of choice for some applications. It can be a time-consuming, expensive process which requires very specialized equipment. Moreover, the underlying surface (typically a bond coat surface) often requires a considerable amount of preparation if it is to be covered with a top coating applied by EB-PVD. For example, U.S. Pat. No. 4,880,614 (Strangman et al) describes the necessary treatment of a bond coat to make its surface very smooth and uniform. Moreover, protective coatings produced by EB-PVD sometimes have a relatively porous, "columnar" structure which allows penetration of various corrosive elements into the metallic bond coat, through fissures between the columns of the deposit. This occurrence can in turn lead to spallation.
Thus, plasma-spray techniques are often a very desirable alternative for applying TBC's to various metal substrates, such as turbine combustor parts. These techniques do not usually require the expensive equipment employed in EB-PVD. Furthermore, plasma spray systems are very well suited for coating large parts, with maximum control over the thickness and uniformity of the coatings. One exemplary TBC which has been of great interest in the turbine engine industry is based on zirconia (ZrO.sub.2), usually stabilized with a compound such as yttria (Y.sub.2 O.sub.3).
While there are obviously advantages to the use of plasma-sprayed coatings, there are also drawbacks in some situations. Some of the disadvantages result from the very high temperatures to which parts such as turbine engine components are exposed during service, as mentioned previously. These temperatures usually promote some oxidation of the various elements in a bond coat like MCrAlY. Typically, alumina becomes the predominant oxidation product originating with the bond coat, and an alumina layer begins to form at the interface of the bond coat and the TBC.
Moreover, the elevated temperatures sometimes lead to undesirable chemical reaction between the bond coat and the TBC--especially when the TBC is zirconia-based and plasma-sprayed. This in turn can lead to a further build-up of alumina and other weaker oxides, causing stresses at the interface between the bond coat and the TBC, and within each coating itself. These stresses can eventually result in spallation within the TBC, which if unchecked, can ultimately cause the TBC to fail and lead to damage of the underlying substrate.
Thus, a method for isolating a plasma-deposited TBC from an underlying bond coat would be welcome in the art. The method should also preferably retard the growth of the above-mentioned alumina layer which results from oxidation. Moreover, the method should still produce a protective coating with all of the desirable performance characteristics of prior art coatings. This is especially true when the substrate is a high performance article like an aircraft engine part.